Since the mid-2000s the goopy blooms, which look mucuslike up close, have cropped up in rivers worldwide. Researchers assumed that the responsible alga, Didymosphenia geminata, was a foreigner or a mutant aggressively invading clean watersheds. But a new analysis suggests that the alga nicknamed didymo is instead native to much of the globe, and that changes in water conditions are to blame for a recent boom in blooms. The controversial suggestion could upend strategies for preventing cases of rock snot.

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